Descend steeply (6)
I believe the answer is:
plunge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">descend steeply</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for plunge that I've seen before include "Dive quickly" , "Thrust into liquid" , "Dive - submerge" , "Dive or swoop" , "Fall suddenly and uncontrollably" .)